This invention relates to a piezoelectric pick-up device for measuring pressures within cylindrical hollow bodies on the basis of their elastic deformation, said pick-up device having a body composed of two parts with a cavity with a circular cross section, in the wall of which there are provided at least two motion transmitting elements and piezoelectric plates.
Hollow bodies, the internal overpressure of which is variable in the course of operation, are generally also stressed, in addition to internal overpressure, by bending stresses and additionally by vibrations. For instance, an injector tube of a diesel engine, on the surface of which tube the course of pressure of the injected fuel is picked-up for diagnostic measurements, forms with the injection pump, the injector, and the engine proper a system oscillating as a unit. Furthermore, the injection tube oscillates, as an elastic element of this system, at its own damped resonant oscillations, which are generated due to quick pressure variations in the tube, the frequency of such oscillations being generally within the limits of 5 to 20 kilocycles.
A number of measuring methos are known for measuring internal overpressures. For diagnostic measurements indirect methods are generally used, such methods consisting in pickingup deformations of an elastic body which depend upon variations of the internal overpressure. Due to their simplicity of design and manufacture, piezoelectric pick-up devices have a high sensibility and a good reproducibility of results. The main drawback of known pick-up devices is the impossibility of eliminating influences due to vibrations and elastic deformations of the measured body, which generate error signals, thereby distorting the results of measuring the internal overpressure of the body.
With known pick-up devices, the variations of the diameter of the tube due to overpressure variations inside the tube are measured at two opposite points. Oscillations due to bending stress of the tube in the course of operation cause variations of the geometric shape of the cross section of the tube; thus the initial circular cross section of the tube changes to an oval one. The pick-up device registers variations of the diameter of the tube which are independent of the overpressure in the tube, that is, parasitic oscillations, and thus yields erroneous signals. Different electric filters, mostly low pass filters are therefore used with said known pick-up devices in order to suppress these parasitic oscillations. The real course of the overpressure in the tube is, however, thus distortes, since the signal then lacks the high frequency components, which are rather important for the analysis of the fuel injection process, for example.